Industrial gas-fired air heater



June 20, 1950 J. A. HARRISON INDUSTRIAL GAS FIRED AIR HEATER 2Sheets-Sheet l Filed March 6, 1947 JNVENTOR. .J4/ves A HAAR/50N Arrow/VsJune 20, 1950 J. A. HARRISON 2,512,326

INDUSTRIAL GAS FIRED AIR HEATER Filed March 6, 1947 2 Sheets-Sheet 2:ELE-E JNVENToR. JAMA-.s A. HAM/sow TIME QTTORNEYS zob Patented June 20,1950 if INDUSTRIAL GAS-FIRED AIR HEATERv James A. Harrison, SouthfieldTownship,

Oakland County, Mich.

Application March 6, 1947, Serial No. 732,746

This invention relates to industrial heating equipment and particularlyto gas-fired heating apparatus for supplying heated air to process ovensor furnaces or for supplying tempered air to factory spaces in order tomake up the air which is exhausted by Ventilating systems.

Heretofore, industrial air heaters of this character have been of twotypes, namely either pullthrough or blow-through. In the pull-throughtype, the air heater is installed on the inlet side of the systems airmoving fan, and in the blowthrough type, when used, the air heater isinstalled on the discharge side of the fan.

Pull-through type heaters have been equipped with atmospheric type'gasburners which for temperature control purposes have turn-down ratioslimited approximately three-to-one. Blowthrough type heaters have beenequipped with blast type gas burner equipment requiring a combustionairblower to deliver the primary air at from 16 ounces to 32 ouncespressure. The turndown ratio of this blow-through type is limited tofrom four-to-one to siX-to-one. Due to the limited turn-down ratio ofthese two types of air heaters, it has been necessary that they bemanufactured with a great number of gas burner sizes in order to answerthe specific capacity needs and still have turn-down available for thecontrol of the temperature of the air being heated. Likewise it has beennecessary that the size of the air heater be quite accurately selecteddue to the limited range in turn-down.

An important object of this invention is to provide an improved airheater which is relatively inexpensive to manufacture and install, ishighly efficient and more economical in operation, and has aconsiderably higher turn-down ratio than heretofore which enables thedelivered air temperature to be accurately set and maintained.

Another important object of this invention is to provide an improved airheater construction including novel means for economically andeiliciently mixing the air and gas to form a combustible mixture, andburning this combustible mixture to heat the air forced through thecombustion zone, with burners interchangeable for wide increases ordecreases in heater capacity.

A further important object of the invention is to provide the air heaterwith burners which will eiliciently operate when air is blown throughthe air heater by the systems ian without the necessityof using aseparate costly combustion airblower and still have ya turn-down ratiofar in excess of that available with existing air heaters,

14 Claims. (Cl. 263-19) A further important object of this invention isto provide an air heater that can be employed in either a blow-throughor pull-through type of system without change other than adjustment.

A further important object of the invention is to provide an air heaterwhich has adjustable air passages so thatnot only the total passage canbe adjusted for air handling capacity but the passages can beindividually adjusted to distribute the air equally on both sides of theflames of combustion.

A further important objectjof thisinvention is to provide an air heaterwith adjustable air passages so that the passages can be' partly closedwhen the air heater is rst lighted and the air passing therethrough iscold and heavy thus eliminating the necessity `of employing an oversizemotor for operating the circulatory air fan.

A stillA further important object ofthe invention is to provide a novelcontrol mechanism for the apparatus which provides a large turn-downratio a'nd enables the apparatus to be adjusted within a line degree ofaccuracy to provide the desired temperature in the chamber, oven orfurnace into which the heated air is directed. Other objects, advantagesand meritorious features will become more fully apparent rfrom thevfollowing specification, appended claims andy accompanying drawingswherein:

Fig. 1 is a schematic view of a system embodying the features of theinvention showing the manner of :heating and circulatingv the airthrough a treatment chamber,

Fig. 2 is a cross sectional view through the air heating apparatus takenalong line 2-2 of Fig. 1,

Fig. 3 is a longitudinal sectional view of the air heating apparatustaken along line 3--3 of Fig. 2, A

Fig. 4 is a schematic view showing the manner of controlling anddelivering gas to the heating apparatus, and

Fig. 5 is a graph illustratingthe operation of the invention.

The invention may be embodied inl various forms and is hereinillustrated as applied to a heat treatment oven or chamber from whichair is drawn and heated and returned to heat the chamber. Referringto'Fig. 1, the heat treatment chamber is generallyindicated at l0 and isprovided with an air inlet I2 at one end and an exhaust stack I4 at theother end. Both the inlet and exhaust openings may be varied in size bysuitable valve control means. A circulatory system is provided forremoving air from one portion of thelchamber, heating it and returningthe heated air to another portion of the chamber. In operation, theinlet and exhaust openings are usually adjusted so that approximately75% or more of the air is circulated through the system, the balancebeing introduced and discharged through the inlet and outlet openings.

The air circulatory system comprises a circular duct I6 generallymounted outside of the chamber and provided with a return air intake I8entering the upper area of the chamber to draw air therefrom. Theopposite discharge end of the duct I6 is provided with an air supplysection 2li, entering another portion of the chamber, and constructedwith a horizontal distributory section 22 having a series of dischargeopenings 24 for distributing the heated air over the lower area of thechamber as shown Iby the arrows in Fig. 1.

Although the duct l and the heating apparatus therein may be disposedinside the chamber, it is preferred for accessibility to locate the ductI6 outside the chamber over the' top as shown or along either side wall.The duct is provided with means in the form of an air blower or fan 26lfor drawing air through the intake i8 and forcing it through the balanceof the system. Provided in the duct l is a slightly reduced circularsection 28 in which-the air heater islocated; In the illustratedembodiment of the invention, the air blower 26- is located on the inletside of the air heater and the system typies a blowthrough type ofapparatus. lit may, however, be locatedvon the outlet side of the casingandV pull air therethrough as in the pull-through types of apparatus. Inthe preferred construction, the air blower functions-todirect the airunder pressure of one to fou-r inches of water column.

The reduced section 2B-is in the form-.of a hollow casing in which. iscontained the air heater. The air heater includes a plurality of gasburner units of similar construction arranged in circularly spacedrelation around the axisor the casing as shown in Figs. 2` and 3. Eachburner. unit comprises a Venturi. tube 30 having the. constrictedportion of' its passage preferably nearer 4 the inlet end than theoutlet end. Air is forced by the blower 26 through the. entire length ofeach venturi, traveling at a greater rate of speed through theconstricted area than any other portion of the venturi. At approximatelythe minimum diameter of the venturi, gas is admitted and mixedA with theair. The gas is. delivered separately to eachfburner unit. by a tube orpipe 32 preferably formed ofk copper. Each tube 32 opens. into theconstricted passage of the venturi of' itsl respective burner unit asshown at 3A. Preferably, the discharge endsection of the gas deliverypipe is wound one or more turns around the Venturi tube as shown at 35to thereby preheat the gas therein beforerit discharges. into theVenturi tube. The purpose or this. is. topreheat the raw gas beforeadmission into, the burner units to thereby proportionately expand thevolume of gas delivered asthe temperature of the air increases. Airforced: through each Venturi tube picks up the-gas admitted thereintoand carries it to the discharge end while it mixes therewith. At thedischarge end of the Venturi tubes, the air'andgas flows therefrom in acombustible stream and in ignited condition for-ms the flame jetsgenerallyv indicated at 36.

Asr previously mentioned, the Venturi tubes arearranged in circularlyspaced relation around the axis of the casing. The seriesfof` Venturitubes are mounted between two substantially coaxial sleeves 38 and 40 ofunequal radii. The outer sleeve 38 is cylindrical and is less than thelength of the casing 28 and preferably located closer to the inlet sideof the casing than the outlet side thereof. The inner sleeve 40 ismounted substantially within the outer sleeve and is provided with aconical section l2 which flares slightly outwardly toward the flame jetarea and then is shaped with a short cylindrical section 44 at itsdischarge end. Both the sleeves project a slight distance beyond thedischarge end of the Venturi tubes as shown in Fig. 2. The inner sleeve48 is opened at both ends and forms a relatively large passage for theflow of air through the duct'system. The outer sleeve 33 is spacedinwardly from the wall of the casing 28 and forms an annular passage d5therearound for the ow of controlled amounts of air therethrough.

On the axis of the casing is a conical shaped member or plug 46,preferably diamond shaped in longitudinal cross section, which ismounted for longitudinal adjustable movement. rIhe plug 46 is mountedwithin a xed subsidiary sleeve 4B supported by brackets or otherwisecentrally Within the casing. The sleeve is preferably provided` withtapering conical end sections lili- 50 which preferably converge towardthe axis of thecasin-g at thesame angle of inclination as the conicalsides of the plug. The plug is xed on an axially extending rod 52 whichis mounted by spacedv supports Sli-54 for longitudinal movement. One endsection of the rod is provided with rack teeth meshing with a pinion 58fixed on a transversely extending control shaft 60. The latter is;provided with any suitable means, such as the hand wheel 52, foreifecting rotation oi the shaft, which will advance or retract the plugrelative to the sleeve 48 to vary the air flow as will be moreparticularly described hereinafter. The maximum diameter of the plugbears such a relation to the sizes of the end openings of the sleeve 48that when adjusted to the extreme positions of its longitudinal movementit substantially closes off the now of air through the sleeve. Movementof theV plug toward closing position at either end of the sleeve willbuild up a pressure differential between inlet and outlet sides of theair heater. The pressure differential or drop should be greater than 1/4inch water column to prevent nash back in the burner tubes 3o.

The annular passage 4,5 formed between the casing 218 and the sleeve 38allows a flow of a oontrolled amount of air from the inlet side to theoutlet side of the heater. This is preferably accomplished by providingan annular plate 64 on the. inlet side having a series oi equally spacedapart openings or ports GB circularly arranged around the axis of thecasing 28 and concentric thereto.y These openings are spaced apart fromoneanother approximately their circumferential dimensions as shown inFig. 2. Overlying the plate. S4 is asecond annular plate 63 which issubstantially thesame size as theformer and similarly provided with aseries. of openings or ports corresponding in size to the openings 66.This second annular plate is mounted for rotatable adjustment relativeto the rst plate from one extreme position in which the openings of thetwo plates. are incomplete registration to the other extreme position inwhich the openings are completely out of registration and no air willflow. therethrough. The plate 58 may be adjusted to 1g any intermediateposition between these extreme positions and thereby regulate the amountof air flowing through the annular passage 45.

The rotatable plate may be either manually' or automatically adjustedand is herein shown as automatically adjustable in response to thetemperature changes in the treatment chamber. As shown in Fig. 2 anadjusting rod 10 is provided for this purpose which extendssubstantially tangentially through an opening 12 in the casing 28 andhas one end pivotally connected as at 14 to the annular plate A68. Theopposite end is connected by linkage, such as that shown at 16 in Fig.1, to an electric motor 18 or other suitable operating device. Thelatter -is provided With a thermal responsive control element 80projecting into the treatment chamber or other heated space and as isconventional practice the control element in response totemperaturechanges opens and closes electrical switches and causes the motor toturn over in one direction or the other and either pull or push thelinkage system and rod to vary the overlapping relationship of theopenings or ports in the two annular plates B4 and 68.

The air passing through the Venturi tubes forms the primary air of thecombustible mixture. The air passing through the annular passage and theannular passages formed by the innermost pair of sleeves 40 and 48 formsthe secondary air of the combustible mixture. By virtue of thisarrangement, secondary air is supplied to both the. outer and innersides of the llames of combustion represented at 36 and an efficientburning operation results. Adjustment of the plug 46 and the size of theports 66 will not only vary the air handling capacity of the heater butvary the distribution of the air so that if desired an equal amount ofsecondary air may be provided on both sides of the burning mixtureissuing from the tubes 30.

Moreover, by adjusting the air handling capacity of the heater it ispossible to reduce the amount of air flowing therethrough when the airis cold and increase the capacity progressively with the increase intemperature. The motor 18 and its thermal responsive control element areso related to the temperature in the treatment chamber that when the airtherein is relatively cold, the size ofthe ports 66 are adjusted toapproximately their minimum size to thereby reduce the volume of airpassing through the heater unit.l As the temperature of the air in thetreatment chamber and in the duct system increases, the motor iscontrolled to open the ports 65 Wider until at the maximum temperaturefor which the apparatus is set is attained at which time the ports willbe opened approximately their widest extent. By vitrue of such acontrol, it is possible to use a low horse power motor for the blowerian 25 because when the air passing therethrough is cold and heavy itsvolume is cut down, but as the air temperature increases and the airbecomes lighter in weight the capacity of the heater increasesproportionately.

The gas delivery tubes 32 Which separately open into the burner tubescommunicate with a common source of gas maintained at the desiredpressure. Referring to Fig. 4, the gas delivery tubes represented by thelines 32 extend to a common control unit which is operable toprogressively increase or decrease the number of lighted burners. Thecontrol unit comprises a circular housing 82 having a rotary valvemember 84 therein. The latter is secured to a shaft 86 which isoperatively coupled to a reversely op- '6 erated electric motor 88 ofthe type shown at 18. The motor 88 in turn Ais controlled by a thermalresponsive control element 89 similar to 88 which extends into thetreatment chamber or is in thermalr'elation to the air therein.

Each gas delivery line 32 has a separate inlet port opening into thehousing 82 as indicated at 90. These openingsor ports are located in oneside Wall 92 of the casing over which the valve member 84 sweeps.` Whenthe valve member overlies a port it cuts off the delivery of gas to theline 32 communicating with this port, The ports 90 are arranged in asemi-circular series and the valve member is generally in the formationof a semi-circular segment adapted in one position, such as that shownin Fig. 4, to overlie and close all the ports with the exception of oneport. The single exception is indicated at 94 and the valve member iscut away as at 96 in order that when the valve'member is in the maximumclosed position shown in Fig. 4 the single port 84 remains open for thedelivery .of gas to burner with which it is connected. This burner willserve as the pilot and be continuously lighted. Suitable stop'means maybe provided on the valve member or the shaft 86 limiting the rotationbetween the maximum closed position shown in Fig. 4 and themaximum openposition Where all the ports`90 are open for feeding gas to all theburners, or the motor 88 may be so controlled as to limit the rotationbetween these two limits. In addition, if desired, individual manuallycontrollable valves may be installed in each gas delivery line 32. Suchvalves are represented at 98 in Fig. 4.

Any suitable detent means may be provided for causing the valve memberto move in step by step manner so that no port is only partially openedand partially closed. As shown in Fig. 4, the side 92 of the housing isprovided with a series of recesses |08 into which a ball ||0 indicatedin dotted outline may drop thereinto. The ball is mounted in a cavity onthe underside of the valve member and is carried around therewith whenthe latter rotates. The balland the recesses are so located with respectto the inlet ports 9D that when the valve member moves to open or closeanother port the ball falls into a recess following complete opening orclosing or the port and thereby resistingly checks further rotation .ofthe valve member.

Raw gas is delivered to the valve control housing 82 by means of adelivery line or pipe which opens into the housing on the side oppositeto the series of inlet ports 90. Preferably this gas is delivered at asubstantially constant pressure, which pressure may be regulated by acontrol device generally indicated at |00. Such device includes thevalve |02 connected by a valve stem to a Apressure responsive membersuch 'as the diaphragm |04. The diaphragm is responsive to differentialpressures exerted against the opposite sides thereof. One side isexposed to the pressure of the raw gasadmitted by valve |02 anddelivered therefrom to the distributing device enclosed within the`housing 82. The opposite side of the diaphragm is exposed to thepressure of air delivered thereto by the conduit |06 which as shown inFig. 1 opens into the air duct It at some point between the fan 26 andthe air heater 28. This vair pressure falls as the temperatureincreases, thus the same falling pressure can be imparted to the raw gasdue to the regulating action of the control device |00.y This airpressure is relatively constant. If the pressure of the raw gas iiowingthrough the control device lull should fall, such as by the opening ofmore inlet portsr 90; the change in the diierential pressures exerted onthe diaphragm will cause the valve IGZ to open further and admit moreraw gas to distributing control device in the housing 82. Should thepressure of the raw gas increase beyond the desired amount, the changein the diierential pressure will lift the diaphragm and cause the valve|02 to move toward closed position thereby reducing the amount of rawgas supplied to the housing 82.

The air heater portion of the apparatus enclosed within the casing 28forms a separate installable unit of theA air circulating duct system.As previously mentioned it may be installed on thedischarge side of airblower and function as a blow-through type of air heater, or it may beinstalled on the inlet side of the systems air moving fan. To facilitateinstallation and removal as a separate unit, the casing 28 of the airheater is provided with outwardly extending'radial flanges {l2-l I2 atthe opposite ends thereof. Each flange is provided with a series ofcircumferentially spaced openings or holes rIlll through which fasteningmeans such as bolts or rivets may extend to secure the unit betweencomparable end portions of the sections making up the duct system I6.The air heater may thus be separately manufactured and installed withoutdifficulty wherever it may be desired in the air circulating system.

The operation of the system is generally understood from the previousdescription. When the system is iirst started up, the air in thechamber, oven or space to be heated is relatively cold and heavy andnormally without any control of the capacity of the air` heater, aconsiderable load will be imposed on the motor driving the air movingfan 26. However, the control motor 'I3 of the present system in responseto the relatively cool temperature of the treatment chamber will haveshifted the annular plate E8 until the openings 66 for the annularpassage 6.5 are substantially closed thereby reducing the volume of aircapable of passing through the air heater. Moreover, the plug 46 may beadjusted by the control 52 to substantially its closed position in thesleeve 48. This will reduce the load on the fan motor thereby enabling amotor of a lower horse power rating to be used. At the same time thecontrol motor 88 in response to the temperature in the treatment chamberwill have moved the valve member 84 in a counterclockwise direction inFig. 4 to substantially the maximum open position. As a resultsubstantially all the burner units 30 will receive raw gas foroperation. The combustible mixture owing from all these burners will beignited by the single continuously operated pilot burner and the airheater will be functioning at substantially its maximum heatingeffectiveness.

As the temperature rises in the treatment chamber or other space beingheated by the system, the control motor 'I8 will function to rotate heannular plate 68 so that the openings 65 are progressively opened inproportion to the rise in temperature. More air is admitted into the airheater increasing the amount of air moving through the air heater. Theconical plug 46 may be adjusted to open the passage through theinnei-most sleeve 43 thereby further increasing the air capacity of theair heater and providing an amount of secondary air approximately equalto thatsupplied by way of the annular passage 5.

8 As the temperature continues to rise the control motor 88r willfunction to rotate the valver member 84 in a clockwise direction in Fig.4 toward its positionsuccessively cutting off the delivery of gas to theburners and thereby reducing in step by step manner the number ofburners in operation.

Ii the controls are set to provide a limit to the rise in temperature inthe heat treatment chamber, for example 450 F., as this temperature isapproached the number of burners in operation will be reduced until thetemperature levels off at this desired limit. The graph in Fig. 5represents the operation of the system from a start in cold condition..Ii' 450F; is the desired temperature, the controls may be set so thatthe teinperature curve begins to level orf atY this degree level. If itshould go above as indicated in the graph, one or more burners will becut oithus reducing the eiectiveness of the air heater. This will causethe tempera-ture of the air in the treatment chamber to fall and if itshould go below the desired degree, one or more burners will bel out in.In actual practice, the temperature curve after approximating thedesired limit will iluctuate slightly above and below the desired degreeas shown in the graph and the cutting in or out of one burner` by thevalve member 84 will suice to hold the temperature at substantially thedesired level.

During the operation of the air heater, the raw gas delivered to theburners will be preheated as it passes through the coils 35 just priorto admission tothe Venturi tubes. This will expand the volume of gasproportionately to the temperature of the air heated. The volume of gasdelivered tothe Venturi tubes therefor maintained proportionate to thevolume of air flowing through Venturi tubes. Since the gas, is admittedinto the Venturi tubes at substantially their point of minimumdia-meter, the action of the air passing these inlet ports 34 induces asuction which obviates the necessity of maintaining the gas under highpressure. The gas need only .be under a suicient amount of pressure tofeed it to the burners.

What I .claim is:

l. An air heating apparatus comprising, in combination, a generallycircular casing, a series of' gas burner units in the form of Venturitubes arranged in the interior of the casing in a substantiallylcircular relation around the axis of the casing, said Venturi tubesoccupying less than the total cross sectional area, of the casing so asto provide one or more air passages between the` Venturi tubes for theuninterrupted ow of air through the casing, means for causing air to beheated; to ilow through the casing from one end to the other endthereof, said Venturi tubes having their-axes extending substantiallyparallel to the axis of the casing so that a part oi the air ilowingtherethrough is caused to flow through the Venturi tubes, means fordelivering gas to each burner unit and causing the gas to enter eachVenturi tube at substantially the minimum cross sectional area thereofso that the gas mixes with the air owing through the Venturi tubes,means responsive to the temperature of the air heatedr by said burnerunits for cutting off the operation of one or more of the burner unitsto prevent increase in the temperature of the air beyond the desiredtemperature level, valve means for one of said air passages adjustableto vary the amount off air iloWing therethrough, and. means responsiveto the temperature of the` air heated by said burner units for adjustingsaid valve means according to the temperature of the air.

2. An air heating apparatus comprising, in combination, a generallycircular casing, a series of gas burner units in the form of Venturitubes mounted in the interior of the casing in a substantially circularrelation around theaxis of the cas-ing and in spaced relation to theinside surface of the casing to form an annular air passagetherebetween, said Venturi tubes having their axes substantiallyparallel to the axis of the casing and their inlet and outlet openingsin corresponding positions in the casing, means for causing air to beheated to flow through the casing at a relatively high velocity from oneend to the other end thereof and causing part of the air to flow throughthe Venturi tubes and another part of the air to flow through theannular pass-age between the Venturi tubes and the inside surface of thecasing, means for delivering raw gas to each burner unit and causing thegas to enter each Venturi tube substantially at the minimum crosssectional area thereof, means responsive to the temperature of the airheated by said burner units for cutting off the operation of one or moreburner units to maintain the air at the desired temperature level,adjustable valve means controlling the amount of air flowing throughsaid annular passage, and means responsive to the temperature of the airheated by said burner units for adjusting said valve means in accordancewith the temperature of the air.

3. An air heating apparatus comprising, in combination, a generallycircular casing, a series of gas burner units in the form of Venturitubes substantially circularly arranged in the interior of the casingbut in spaced relation to the axis of the casing and the inside Wall ofthe casing so as to form ,an axial air passage and an annular airpassage, said Venturi tubes having their respective passages extendingsubstantially parallel to ther axis of the casing, means for causing airto be heated to flow through the casing from one thereof to the otherand to flow through said axial and annular passages and the passages ofsaid Venturi tubes, means for separately delivering gas` to each burnerunit and causing it to enter the passage of each Venturi tube atsubstantially the minimum diameter thereof, means responsive to thetemperature of the air heated by the apparatus for varying the number ofburner units in operation in order to maintain the lair at the desiredtemperature level, adjustable valve means for said annular passage and aseparate adjustable valve means for said axial passage adapted uponadjustment to vary the amount of vair owing therethrough, and meansresponsive to the temperature of the air heated by the apparatus foradjusting at least one of said valve means so as to vary the amount ofair flowing through the passage controlled thereby in direct proportionto the temperature f the all'.

4. An air heating apparatus for installation in a circulatory air systemcomprising, in combination, .a generally circular casing having means onthe opposite ends thereof for securing the same in the system, aplurality of burner units in the form 'of lVenturi tubes arranged in acircular series in the interior of the casing in spaced relation to theaxis thereof and the inner surface thereof to form an axial air passagevand an annular air passage therebetween,

said Venturi tubes being opened ended and having their respectivepassages extending substantially parallel to the axis of the casing, agas delivery pipe for each burner funit adapted to communicate with anoutside source of gas and opening into the Venturi tube passage of itsburner unit at substantially the minimum cross sectional area. thereof,each gas delivery pipe being coiled one or more times about its burnerunit prior to opening into the Venturi pass-age thereof, adjustablevalve means for each of said axial and annular passages, and means forcontrolling the adjustment of said valve means in order to vary the aircapacity of the heating apparatus.

5. An air heating unit adapted to be installed in a duct of an aircirculating system comprising, in combination, a generally circularcasing having means on the opposite ends thereof for securing the sameto the open ends of two separated sections of an air duct, a pair ofsleeves of different diameters mounted in the easing concentric with theaxis thereof and forming an axial air passage through the inner sleeveand an annular air passage between the outer sleeve and the casing, aplurality of gas burner units arranged in circularly spaced apartrelationship in the area between said sleeves, each burner unit havingan open ended Venturi passage extended substantially parallel with theaxis of the casing, a gas delivery pipe for each burner unit adapted tocommunicate with an outside source of gas and having its discharge endopening into the Venturi passage of its respective burner unit atsubstantially the minimum diameter thereof, each gas delivery pipe beingcoiled one or more turns about its respective burner unit for preheatingthe gas prior to discharge into the Venturi passage, valve control meansfor varying the amount of air adapted to be passed through said annularpassage, valve control means for varying the amount of air adapted to bepassed through said axial passage, and control means operable fromoutside of said casing for adjusting said two valve means.

6. An air heating apparatus comprising, in combination, an elongatedgenerally circular casing, a series of gas burner units in the form ofVenturi tubes shorter in length than the length of the casing, meansmounting the Venturi tubes in the interior of the casing in spacedrelation to the opposite ends thereof and with the axes of the Venturitubes extending parallel to the axis of the casing and their inlet andoutlet openings in corresponding positions in the casing, said mountingmeans further disposing the Venturi tubes in spaced-relation to theinside surface of the casing to form an annular air passagetherebetween, means for ilowing air to be heated through the casing at arelatively high velocity from on-e end to the other end of the casingand causing a part of the air to 'ow through the Venturi tubes andanother part of the air to iiow through the annular passage between theVenturi tubes and the inside surface of the casing, means for deliveringgas to each burner unit and causing the gas to enter each Venturi tubesubstantially at the minimum cross sectional area thereof, and valvemeans in said annular passage controllable from outside of the casingfor varying the volume of air flowing through the passage. l l 7. An airheating apparatus comprising, in combination, a generally circularelongated casing, a plurality of gas burner units in the form of openended Venturi tubes shorter in length than 11 the length of the casing,means mounting the Venturi tubes in the interior of the casing in acircular series of less radius than the casing and forming a centralpassage on the axis of the casing and an annular passage between theVenturi tubes and the inner surface of the casing, said Venturi tubesbeing mounted in the same plane in the casing in spaced relation to theopposite ends of the casing and with their axes extending substantiallyparallel to the axis of the casing and with their inlet and outletopenings in corresponding positions in the casing, means for flowing airto be heated through the casing from one end to the other end thereofand causing a portion of the air to flow through the Venturi tubes andportions of the air to flow through the central and annular passages oneither side of the Venturi tubes, means for delivering gas to eachburner unit from outside of the casing and causing the gas to enter eachVenturi tube substantially at the minimum cross sectional area thereof,and separate adjustable valve means in said annular passage and in saidcentral passage for varying the volume of air flowing therethrough.

8. In a substantially closed circulatory air heating system including asparts thereof a chamber to be heated and a conduit opening at itsopposite ends into communication with spaced areas of the chamber, asection of the conduit forming an air heater and provided in theinterior thereof with a plurality of open ended tubular gas burnershaving their axes extending parallel to the axis of the section andoccupying an area less than the total cross sectional area of thesection to provide for the flow of air therearound, means for moving airthrough the system in one direction and causing the air passing throughthe air heater to divide and ow both through and around the gas burners,means for progressively increasing or decreasing the number of the gasburners in operation, and means responsive to the temperature of the airin said chamber for governing the action of said second mentioned means.

9. In a substantially closed circulatory air heating system including asparts ofthe circulatory system a chamber to be heated and a conduitopening at its opposite ends intocommunication with spaced areas of the-chamben'a section of said conduit forming an air lheater and having aplurality of open ended tubular gas burner units mounted inthe interiorof the section and arranged in a group spaced radially inwardly of thesurrounding wall portion of the section to form an annularly shaped airpassage therebetween, an air impelling device adjacent to one end of theair heater operable to move air through said section and through saidburner units and through said annular passage, means for delivering gasto said burner units from an outside source of supply to mix with theair flowing therethrough to form a combustible mixture, an adjustablevalve in said annular passage for varying the volume of air flowingtherethrough, means responsive to the temperature of the air in saidchamber for lvarying the number or' burners in operation to maintain theair at the desired temperature, and .separate means responsive to thetemperature of the air in the chamber for adjusting said valve.

10. An air heating apparatus comprising, in combination, a generallycircular casing, a plurality of open ended tubular gasburner units inthe interior of the casing having their axes extending substantiallyparallel to the axis of the casing and arranged :in `a group spacedradially inwardly on all sides thereof from the surrounding Wall portionof the casing to form an annularly shaped air passage therebetween,means for iiowing air to be 'heated through the casing to thereby causea part of such air to ow through the burner units land another `part ofsuch air to ow through the annular passage, means for delivering gas toeach burner unit to form a combustible mixture with the air ilowingtherethrough, a .pair of a-nnularly shaped plates disposed in saidannular air passage in juxtaposition to one another, said plates eachhaving a series of similarly shaped apertures spaced apart from oneanother approximately their'respective circumferential dimensions, andmeans operable from outside of-the casing for -circularly adjusting oneof said plates relative to the other plate from a position in which theAapertures of the plate substantially register with one another to aposition in which the apertures are substantially out of registrationwith one another in order to vary the volume of air `flowing through theannular passage.

11. An air heating apparatus comprising, in combination, a generallycircular casing, a pair of open ended sleeves of -diierent-diametersdisposed inside of the casing `in Aco-afxial relationship to one anotherand to the axis of the casing, said sleevescooperating with the casingto form an annularly shaped air passage between the larger diametersleeve and the casing and a central passage through the smaller diametersleeve, a plurality of tubular burnerunits mounted in the space betweensaid sleeves in circularly spaced apart relationship around the axis ofthe casing land having their respective axes extending parallel to theaxis of the casing, means for delivering gas from a source of supplyoutside of the casing to each burner unit for mixture with the airtherewithin, an air impelling device at one end of fthe casing operableto force air under relatively Vhigh velocity `through the casing andthrough said burner units and said annular and central passages, andmeans for adjusting the cross section areas of said annular and centralpassages ito Yvary the volume of air owing therethrough.

l2. In a gas-:red air heating system, the method of operating the systemwhich comprises continuously circulating the air to be heated in amoving streamthrough the system, causing the air stream of thesystemtoflow fthrough a combustion zone, `introducing raw gas into thecombustion Zone at 4a 'plurality `of points separated from one anothertransversely'of the air stream flowing therethrough, utilizing 'the airin the air stream as lthesole source of primary and secondary-airfor thecombustible mixture by dividing the vstream adjacent to-each Vpoint intoan inner *core of 'primary -air for vimmediate mixture with the raw gasintroduced aft-that point and into anlouter vlayer of secondary air forlater mixture with the gas and primary air further along the stream,varying the volume of the air stream passing through the combustion zonein direct proportion to the temperature of the air stream, andvarying-the amount of raw gas introduced into the combustion zone ininverse proportion to the temperature of the air stream.

13. In a circulatory-air heating system including a-combustion zonein-Which the air in the system is heated and-afchamber which is heatedby theair inthesystem, the method of operating the system whichAcomprises continuously moving an air stream under forced draft throughthe system in one direction, introducing iuel from an outside source ofsupply into the combustion zone, utilizing the air stream of the systemflowing through the combustion zone as the sole source of air forforming a combustible mixture with the fuel introduced into thecombustion zone, varying the amount of fuel delivered to the combustionzone in inverse proportion to the temperature of the air in the chamber,and varying the volume of air owing through the combustion zone indirect proportion to the temperature of the air in the chamber.

14. In a circulatory air heating system including a chamber to be heatedby air and a combustion zone for heating the air, the method ofoperating the system which comprises continuously circulating an airstream under forced draft through the system in one direction,introducing a gaseous fuel from an outside source of supply into thecombustion zone, utilizing the air stream of the system flowing throughthe combustion zone as the sole source of air for forming a combustiblemixture with the gaseous fuel thus introduced, controlling the amount ofgaseous fuel delivered to the combustion zone in inverse proportion tothe temperature of the air in the cham- 14 ber, and controlling thepressure of the gaseous fuel thus introduced in accordance with thepressure of the air stream flowing through the combustion zone.

JAMES A. HARRISON.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,349,668 Good Aug. 17, 19201,958,913 DeCoriolis et al May 15, 1934 2,000,733 Avery May 7, 19352,097,544 Ames Nov. 2, 1937 2,120,803 Grant June 14, 1938 2,147,568Barber Feb. 14, 1939 2,383,641 Focke Aug. 28, 1945 2,403,230 Nagel July2, 1946 2,432,525 Kruse Dec. 16, 1947 2,447,482 Arnold Aug. 24, 1948FOREIGN PATENTS Number Country Date 120,663 Great Britain Nov. 21, 1918316,667 GreatI Britain May 22, 1930

